Wednesday, November 16, 2011

فيزياء الجسيمات الاولية

The four fundamental forces of nature are the electromagnetic force, the strong force, the weak force and the gravitational force. The non-gravitational forces are described by quantum Yang-Mills field theories on a flat Minkowski four dimensional spacetime and they are unified in the standard model of particle physics.

Particle physics also called high energy physics is a major branch of physics which studies elementary constituent of matter and their interactions. Here we mean by elementary particles those particles which are not bound states of other more elementary particles such as the electron and the photon. The proton for example is not an elementary particle but a bound state of more elementary particles known as quarks.

Matter within the standard model is described by spinorial fields with the exception of the scalar Higgs particle which is not discovered yet. Forces (corresponding to radiations) are described by vector gauge fields. They are mediated by gauge bosons which are the photon (electromagnetic), the gluons (strong) and the W^+,W^- and Z^0 (weak). Electromagnetic and weak interactions are unified within the electroweak theory with the symmetry between them  only spontaneously broken via the Higgs. The strong force is described by quantum chromodynamics or QCD. The unification of the electroweak theory and QCD is the standard model.

Gravity can be unified with the other  forces only within the context of string theory which is not a field theory. Classical gravitation is described by general relativity. Modern cosmology is based on the combination of the principles of general relativity and particle physics and is sumarized in the so-called standard model of cosmology. The most important problems of cosmology are dark matter and dark energy. The fundamental solutions of these problems  will lie at the end within the theory of quantum gravity. The only known successful theory of quantum gravity is again given by string theory under the name of M-theory. This is supposed/claimed to be a theory of everything.

Some of the ingredients of string theory are supersymmetry (which the LHC particle accelerator is  currently aiming  at discovering with the Higgs particle of the standard model), Kaluza-Klein extra dimensions, conformal field theory, Yang-Mills gauge theories and Riemann surfaces together with a lot of pure mathematics. I can safely claim that this is the most challenging subject that one can study in theoretical physics at this current point of history.

Saturday, November 5, 2011

المادة المظلمة و الطاقة المظلمة

It is well established by recent observations that the universe is made up of 74 per cent dark energy and 22 per cent dark matter while the rest is ordinary baryonic matter.  The nature and origin of dark energy and dark matter are still hotly debated. This is a vast interdisciplinary topic which is of paramount importance to modern particle physics, cosmology  and astrophysics. 

The most important model for dark energy is the cosmological constant. This is the energy of the vacuum in the presence of gravity. From the more fundamental particle physics (field and string theories) point of view the basic problem of dark energy and dark matter is reducible (at least in principle) to the problem of quantum gravity. At the current state of affairs only string theory has a quantum theory of gravity and thus can potentially address this problem in a consistent way. Unfortunately this is not easy to do in practice and thus we should content ourselves (at least at this stage) with simpler approaches. It seems that supersymmetry (or more precisely supergravity) is in all cases the first step in the right direction. We should also mention here that there is also a new proposed theory of quantum gravity known as Horava-Lifshitz gravity which is for many reasons more realistic than string theory.

Supersymmetry, now we mean minimal supersymmetric standard model (MSSM), is also potentially relevant to the issue of dark matter.  Although dark matter does not emit or absorb electromagnetic radiations at all energy scales its gravitational interactions dominate on all cosmological scales.  An excellent candidate for dark matter is neutralino which is the lightest supersymmetric stable particle formed from a linear combination of the supersymmetric partners of the photon, W^0 and the Higgs boson. Let us recall that in any supersymmetric theory to each boson we associate a fermion and to each fermion we associate a boson. In other words every particle has a super partner with the same mass but with a spin which differ by half a unit. As it turns out the neutralino can interact weakly with ordinary matter and it has mass of the order of Tev with a cosmological abundance equal Omega=1 in the current epoch of the universe and thus it can be identified with dark matter in the universe.